First check your brake fluid. This has been known as a warning for improper fill.

Most likely after that you have a bad sensor somewhere. You can try checking all of them, replacing them and what have you. I doubt new rims size would afect ABS although I can not say for certain. Try checking all the ABS lines to make sure there are no kinks or anything either and the lines are fine as well.

Other than that you can find an ABS scanner which should tell you exactly what the problem is. To buy the ABS scanner is costly, and I don't think auto part stores do it for free like they do with CEL. It might be worth checking out though.

Not going to lie I'm not 100% sure. The sensors should be right behind the brake caliper somewhere. You should see a second line coming in under the wheel well and I believe the sensor is at the end.

I'm not sure of this or the price because I haven't had the chance to mess with ABS system (thank god, *knock on wood*). To replace them I'd observe some extra safety rules (actually disconnecting the battery, maybe even as faw as bleeding the system). When I get home I will see what the Haynes manual has to say. If I forget to post again by tomorrow and you're still unsure just bump this thread and it'll remind me.